Electrolytic capacitors are increasingly being used in the design of circuits due to their volumetric efficiency, reliability, and process compatibility. Typically, electrolytic capacitors have a larger capacitance per unit volume than certain other types of capacitors, making electrolytic capacitors valuable in relatively high-current and low-frequency electrical circuits. One type of capacitor that has been developed is a wet electrolytic capacitor that includes an anode, a cathode, and a liquid or “wet” working electrolyte. Wet electrolytic capacitors tend to offer a good combination of high capacitance with low leakage current. In certain situations, wet electrolytic capacitors may exhibit advantages over solid electrolytic capacitors. For example, wet electrolytic capacitors may, in certain situations, operate at a higher working voltage than solid electrolytic capacitors. Additionally, by way of example, wet electrolytic capacitors may be much larger in size than solid electrolytic capacitors, leading to larger capacitances for such large wet electrolytic capacitors.
In conventional wet electrolytic capacitors, the anode may be a metal foil (e.g., aluminum foil). Because the electrostatic capacitance of the capacitor is proportional to its electrode area, the surface of the metallic foil may be, prior to the formation of the dielectric film, roughened or subjected to a chemical conversion to increase its effective area. This step of roughening the surface of the metallic foil is called etching. Etching is normally carried out either by the method (chemical etching) of conducting immersion into a solution of hydrochloric acid or by the method (electrochemical etching) of carrying out electrolysis in an aqueous solution of hydrochloric acid. The capacitance of the electrolytic capacitor is determined by the extent of roughing (the surface area) of the anode foil and the thickness and the dielectric constant of the oxide film.
Due to the limited surface area that may be provided by etching metallic foils, attempts have also been made to employ porous sintered bodies, also called “slugs”, in wet electrolytic capacitors. A tantalum slug, for instance, may be formed by mixing powdered tantalum particles with a suitable binder/lubricant to ensure that the particles will adhere to each other when pressed to form the anode. The powdered tantalum is compressed under high pressure around a tantalum wire and is sintered at high temperature under vacuum to form a sponge-like structure, which is very strong and dense but also highly porous. The porosity of the resulting tantalum slug provides a large internal surface area. Despite its high surface area, however, anode slugs may present high ESR and sensitivity of the capacitance to frequency. Further, the slugs are typically larger in size than the anode foils, thus making it difficult to incorporate them into application in which high volumetric efficiency is needed. As such, a need currently exists for an improved wet electrolytic capacitor.